1. Field of the Invention
The present invention relates to a mechanism for preventing eccentricity of a pair of relatively rotating lens-supporting rings.
2. Description of the Related Art
In a zoom lens system of the prior art, there are contradictory demands for a high zoom ratio and miniaturization. For example, in a two-lens-group zoom lens system which can be miniaturized, if the paths of the lens groups thereof (hereinafter, zoom paths) are determined (i.e., solutions for zoom paths) so that the zoom ratio becomes high, interference of the lens groups occurs on the telephoto side, or interference of a lens group and the image plane occurs on the wide-angle side. On the other hand, in a three-lens-group zoom lens system, the zoom ratio thereof can be made higher than that of a two-lens-group zoom lens system; however, miniaturization is difficult. Furthermore, if an attempt is made to obtain a higher zoom ratio, and if the power of these three lens groups is determined accordingly, precision on the operations of the zoom lens system cannot be obtained due to the mechanical structure thereof.
The assignee of the present application has proposed an unprecedented zoom lens system that meets the contradictory demands of high zoom ratio and miniaturization (U.S. patent application Ser. No. 09/534,307, U.S. Pat. No. 6,369,955 Japanese Patent Application No. Hei 11-79572). This zoom lens system has the following characteristics: it includes a plurality of movable lens groups for varying the focal length; at least one of the lens groups is a switching lens group which includes two sub-lens groups, one of the sub-lens groups being a movable sub-lens group that can be selectively positioned at either one movement extremities in the optical axis direction with respect to the other sub-lens group; the movable sub-lens group of the switching lens group is positioned at an extremity of a short-focal-length zooming range, from the short focal length extremity to an intermediate focal length, and at the opposite extremity of a long-focal-length zooming range, from the intermediate focal length to a long focal length extremity; and zoom paths of the switching lens group and the other lens groups are discontinuous at the intermediate focal length and are defined to focus on a predetermined image plane corresponding to the position of the movable sub-lens group. There may be one or more intermediate focal lengths.
In a switching lens group frame of a zoom lens system having the switching lens group as described above, a mechanism is required to support a pair of lens-supporting rings, each of which supports one of the two sub-lens group in a manner that allows the pair of lens-supporting rings to move toward, or away from, each other. In such mechanisms, it is desirable that eccentricity (decentration) between the pair of the lens-supporting frames be eliminated (in other words, optical axes of the two lens-supporting frames should be aligned with one another) when the pair of lens-supporting rings are in a position where the pair of lens-supporting rings are as closely positioned to each other as possible (i.e., a mutually close position) or in a position where the pair of lens-supporting rings are positioned as far apart from each other as possible (a mutually distant position).